Modern semiconductor technology is highly dependent on an accurate control of the various processes used during the production of integrated circuits. Accordingly, the wafers are inspected repeatedly in order to localize problems as early as possible. Furthermore, a mask or reticle is also inspected before the actual use during wafer processing in order to make sure that the mask accurately defines the respective pattern. The inspection of wafers or masks for defects includes the examination of the whole wafer or mask area. Especially, the inspection of wafers during the fabrication includes the examination of the whole wafer area in such a short time that production throughput is not limited by the inspection process.
Scanning electron microscopes (SEM) have been used to inspect wafers. The surface of the wafer is scanned using, e.g., a single finely focused electron beam. When the electron beam hits the wafer, secondary electrons and/or backscattered electrons, i.e. signal electrons, are generated and measured. A pattern defect at a location on the wafer is detected by comparing an intensity signal of the secondary electrons to, for example, a reference signal corresponding to the same location on the pattern. However, because of the increasing demands for higher resolutions, scanning the entire surface of the wafer takes a long time. Accordingly, using a conventional (single-beam) scanning electron microscope (SEM) for wafer inspection is difficult, since the approach does not provide the respective throughput.
Wafer and mask defect inspection in semiconductor technology needs high resolution and fast inspection tools, which cover both full wafer/mask application or hot spot inspection. Electron beam inspection gains increasing importance because of the limited resolution of light optical tools, which are not able to handle the shrinking defect sizes. In particular, from the 20 nm node and beyond, the high-resolution potential of electron beam based imaging tools is in demand for detecting all defects of interest.
Current multi-particle-beam systems may include an aperture lens array. The focal length of an aperture lens is inversely proportional to the difference of the electric field component (along the average axis) before and after the aperture. By shaping the field distribution along an aperture lens array, the focal length of the individual apertures can be varied in such a way that the field curvature of the beamlets can be controlled (or corrected). In such a configuration, other off-axial aberrations (field astigmatism, off-axial coma, and distortion) remain. To mitigate these remaining aberrations, the intermediate beamlet foci are often strongly magnified images of the source. The images of the source are strongly demagnified with the downstream objective lens. This tradeoff between the demagnification and remaining off-axial aberrations limits the performance of such devices. Another way, which is often employed, is to limit the total emission angle from the source (i.e., the number of total beamlets), so that the off-axial aberrations can be reduced.
In view of the above, a charged particle beam device and a method of imaging a specimen with an array of primary charged particle beamlets is provided that overcome at least some of the problems in the art.